Oxide-dispersion-strengthened (ODS) high temperature alloys offer combinations of high-temperature strength, oxidation resistance, and hot corrosion resistance that can not be obtained from other alloys. For example, a nickel-based alloy with yttrium oxide dispersoids, INCONEL alloy MA 754 has been considered as one of the ODS alloys with greater potential in the next generation advanced gas-turbine hot section components such as turbine vanes and combustor liners. In such applications, the ODS alloys are typically used to form LAMILLOY sheets. As is known in the art, LAMILLOY is a multilayered porous material designed for cooled airframe and propulsion system components. It features a labyrinth of holes and passages in a laminated assembly. LAMILLOY is produced by photochemical machining an array of pedestals and holes in two or more layers of sheet material and subsequently diffusion bonding the layers into the laminated sheet configuration.
Many applications require that ODS alloys be joined to either themselves or to other materials. Because welding processes utilize melting of the base material, they are unsuitable for use with the porous LAMILLOY material due to destruction of the porous structure at the weld area. Furthermore, MA 754 cannot be successfully joined by a fusion (arc) welding process while retaining any significant percentage of the alloy's stress rupture strength. Fusion welding causes dispersoid agglomeration and the development of weld solidification grain boundaries transverse to the rolling direction of the material. This results in a loss of high-temperature strength and oxidation resistance.
The joining of two LAMILLOY sheets is therefore accomplished by brazing, which is performed at a temperature below the melting point of the base material. The brazing process requires a lap joint (overlapping ends of the material to be brazed) instead of the butt joint normally used in welding. An example of an application where it is desirable to join two LAMILLOY sheets together by means of brazing is in the formation of vanes 10 for use in a turbine engine, as illustrated in FIGS. 1-4. After forming the material into the vane 10 required shape (design dimensions), the trailing edge 12 must be brazed in order to constrain this region to the design dimensions.
The technical challenges involved in brazing the trailing edges 12 of the MA 754 LAMILLOY vanes 10 include the high residual stress in the LAMILLOY, the very complex contour of the vanes 10, the extremely thin dimension (0.002 to 0.004" thick) of the LAMILLOY cover sheets at the braze joints, and the LAMILLOY holes adjacent to the joints.
The high residual stress is generated during the LAMILLOY vane forming operations. This residual stress creates a large force on the tailing edge joint 12 that results in forcing the joint open during the braze operation. The brazing is accomplished by placing a braze material foil into the gap in the trailing edge 12, using laser tack welds to hold the two halves of the gap together until the brazing is complete. The brazing is then performed at a temperature of approximately 2300.degree. F. As indicated in FIG. 3 by the arrows 14, the maximum gap in the trailing edge 12 joint in the area of the braze is desired to be 0.002-0.004 inch maximum.
The trial brazing of a test vane indicated that laser tack welds used in brazing MA 754 sheets were not strong enough to maintain the braze joint gap of 0.002 to 0.004 inch. Some of the laser tack welds broke during the brazing thermal cycle. Metallographic evaluation showed the actual maximum gap 14 in the braze joint was as wide as 0.020 inch. Wide gap brazing (0.08-0.020 inch) often requires multi-brazing (re-brazing) thermal cycles and application of more braze filler metal, which causes more braze erosion of MA 754 base material. Additionally, more braze filler metal applied for wide gap brazing makes it difficult to prevent excessive braze material from flowing into the LAMILLOY holes and passages at the joint 12. This is because holes in the surface of the LAMILLOY material are within 0.010 inch from the braze area.
Additionally, the LAMILLOY cover sheets at the braze joints are extremely thin (0.002 to 0.004" thick), therefore the brazing process window (brazing temperature and time) is very narrow. It is very crucial to precisely control the brazing temperature and time, otherwise the braze filler metal could completely erode the thin LAMILLOY cover sheet and flow into LAMILLOY cooling channels. This will ruin the LAMILLOY components. Conversely, strong braze joints will not be achieved with insufficient brazing temperature and time. This is illustrated in FIG. 4, which is a cross-sectional end view of the braze joint area 12, showing the braze material 16 and the LAMILLOY holes 18. The dimension 20 shows that the holes 18 are generally only 0.002 to 0.004 inches from the braze material 16. Even though the braze temperature is approximately 200.degree. F. lower than the LAMILLOY melting temperature, the base metal can still erode (dissolve) in the very thin region. Such interalloying can reduce the base metal thickness until the braze material is allowed to run into the holes 18.
There is therefore a need for a means for brazing a joint in an oxide-dispersion-strengthened (ODS) high temperature alloy workpiece where stresses in the workpiece operate to open the joint during the brazing process. Furthermore, such means for brazing must allow the required brazing time and temperature profile to be maintained in order to minimize erosion of the base metal. The present invention is directed toward meeting this need.